The present invention relates-generally to meters for measuring the flow of a fluid through a conduit comprising a laminar flow element for developing a pressure differential across a sensor that varies in accordance with fluid flow in the conduit and more particularly to a wide-range fluid flow measuring device comprising a laminar flow element which maintains low Reynolds numbers through a wide flow and pressure range.
Laminar flowmeters use the linear relationship between the fluid flow rate and static pressure drop or heat transfer. The criteria for the mechanical design of a flow restriction that creates and maintains laminar flow through a measuring device is based on studies conducted by Osborne Reynolds in 1883 relating to fluid flow properties through pipes of different diameter as well as the determination of boundary layers where the transition between laminar flow to turbulent flow occurs. The regimes where laminar or turbulent flow prevails are prescribed by a dimensionless parameter known as the Reynolds number, defined as D.sub..rho. V/.mu., where V is the mean velocity of the fluid, D is the characteristic linear dimension of the pipe, .rho. is the fluid density, and .mu. is the absolute viscosity of the fluid. Generally, laminar flow occurs at Reynolds numbers of less than 2,000, but in practice Reynolds numbers of less than 1,000 are used to ensure laminar flow under all conditions (e.g., viscosity and density variations with temperature).
Once the above condition is met, the advantages of laminar flow can be utilized in providing a flowmeter that has a wider range of operation than non-linear flowmeters. The linear relationship that exists between pressure drop and flow rate can be utilized by both pressure-type and thermal-type flowmeters.
Typically, flowmeters employ a laminar flow element to develop a pressure drop in a shunt path around the laminar flow element thereby to effect flow measurement.
One example of such device is disclosed in U.S. Pat. No. 3,838,598 issued to Tompkins. Tompkins teaches the use of plurality of capillaries to create laminar flow through the measuring device and the use of differential static pressure information to calculate the volumetric flow rate. Certain drawbacks exist with respect to such design. In order to provide flowmeters that have different full scale flow rates, different laminar flow elements must be provided for each flow rate. The capillaries that form the flow passage are also susceptible to clogging if large particles are present in the flow stream. Clogging of the flow capillaries results in a change of pressure across the element for a given flow rate, and thus recalibration of the instrument is necessary.
A different type of laminar flow element of substantially rectangular cross section is proposed in U.S. Pat. No. 4,118,973 issued to Tucker et al. In Tucker, rectangular grooves are machined into plates. By varying the width of the channels and/or by stacking a number of plates, the effective cross section of the flow passage can be varied while low Reynolds numbers are maintained.
Alternative designs are proposed in U.S. Pat. No. 4,497,202 issued to Mermelstein and U.S. Pat. No. 4,427,030 issued to Jouwama. While rectangular laminar flow elements are relatively easy to manufacture and lend themselves to an accommodation of full scale flow rate during assembly, the problem of precision and repeatability remains a difficult task to achieve.
Moreover, Baker, et al, U.S. Pat. No. 3,443,434, entitled "Fluid Flow Measuring Apparatus," discloses a flow measuring device comprising a restrictive element that is interposed in a main line to develop a differential pressure therein. A heated shunt path is connected around the element so as to divert a portion of the fluid from the high pressure side of the restrictive element, through the shunt to the low pressure side of the restrictive element. A thermal transducer is connected in the shunt path to measure the temperature gradient caused by fluid flow through the heated shunt circuit. Temperature variations sensed by the transducer provide an indication of mass flow in the shunt path which is a small but precise fraction of the total flow in the main line.
Weichbrod, U.S. Pat. No. 3,071,160 discloses a laminar flow element exhibiting a linear pressure drop related to flow rate. The element comprises smooth flat sheets and rectangularly indented sheets laid together in pairs to develop slots of a thickness smaller than the width. Specifically, the flow channels are of substantially uniform depth in the range of 0.002 to 0.100 inch and of a width at least ten times the depth. The paired sheets are spirally wound upon mandrel to a desired diameter.
The problem associated with all such known laminar flow elements is that metering of different fluids and flow rates requires a change in area of the laminar flow element which is relatively expensive in that significant tooling is involved.
Moreover, precision laminar flow elements have heretofore been difficult to manufacture because their critical dimensions must be maintained at tight tolerances. In the case of flow elements of rectangular cross section, the critical dimension is the depth of the channel. Pressure drop across a rectangular laminar flow element changes with the third power of the depth. This means that a change of 10% in the depth of a channel that is 0.010" deep results in about 30% change in pressure drop for the same flow rate.
Consistent pressure drop across similar flow elements is an important requirement for mass production of a device. It greatly reduces the amount of time that is required for calibration of the device and results in repeatable and identical performance of different units. One example in which such repeatability is required is in systems where wide changes in flow impedance can adversely affect constant flow maintenance due to limitations of the pump. Replacement of a flowmeter in such a system should produce very little effect, if any, in the flow rate without changing the pump settings. While the inconsistency in producing identical pressure drop can be adjusted electronically to produce the same level of output signal, higher noise levels and the difficulty associated with zero stability of most sensors dictate a somewhat limited range of amplifier gains.
Mechanical adjustment of a laminar flow element for producing equal pressure drop for identical units is preferred over electronic output adjustment for the reasons presented above. One example of such adjustment means is disclosed in my U.S. Pat. No. 5,297,427. In this design, an adjustment screw moves a plate inside a rectangular flow channel, changing the effective width of the flow passage. Another such example is disclosed by Drexel in U.S. Pat. No. 4,524,616, where an adjustment screw moves a frusto-conical flow element inside a mating bore.
While mechanical adjustment of a laminar flow element may be practical for producing consistent pressure drops, the high cost of manufacturing such devices may be prohibitive in the highly competitive low cost flowmeter market. The number of components involved in adjustable flow elements as well as the time spent on adjusting the element during calibration substantially increases the cost of such flowmeters.
The aforementioned problems associated with the production of precision laminar flow elements emphasizes the need for a rugged, low cost, precision flow element. Accordingly, the broad object of the present invention is a precision laminar flow element that can be mass produced at relatively low cost.
Another object of the present invention is a laminar flow element that exhibits repeatability of pressure drop without the need for mechanical adjustment.
Yet another object of the invention to provide a simple method of producing a series of different laminar flow elements covering a wide range of full scale flow rates, while maintaining the same full scale pressure drop for use with identical electronic circuit settings.